This invention relates to expander circuits in multi-channel amplifier systems used commonly, among other possible reasons, to re-establish the desired dynamic quality of the original signal components altered by compression and peak limiting operations carried out during a recording operation. A stereo sound system drives a left and a right speaker unit with left and right channel signals comprising what can be referred to as apparent monaural image-producing signal components having subsantially identical wave forms (in terms of amplitude and shape) and stereo image-producing signal components having substantially different wave forms. The substantially identical monaural image-producing left and right channel signal components produce an apparent sound source (which can be referred to as an apparent monaural sound image) midway between spaced speaker units. A significant percentage of stereo recordings surprisingly produce such an insufficient apparent monaural sound image that the center area in front of a listener positioned not far from the speaker appears to be acoustically weak. A stereo sound image is produced by these spaced speaker units when substantially different stereo image-producing signal components are fed to these spaced speaker units. In such case, an apparent sound image therefrom appears to be located substantially to one side or the other of the midway point between the speaker units. The ratio between the amplitudes of stereo image-producing signal components reproduced by these spaced speaker units determines the apparent location of the stereo sound image perceived by the listener. If a given stereo image-producing signal component is only reproduced by the left speaker unit, the apparent location of the stereo sound image produced thereby will be to the far left, whereas if the same signal component is also reproduced by the right speaker unit but to a lesser degree (as is commonly the case since complete stereo signal separation is uncommon), the apparent location of the stereo sound image is closer to the midway point between the speaker units in proportion to the degree to which the wave forms of the left and right stereo image-producing signal components approach near equal values.
Almost all recordings have their dynamic qualities alterned to some degree by means of compression and peak limiting. Therefore, for the most accurate sound reproduction expansion in the playback system is needed to re-establish the originally recorded signals. In a stereo playback system consisting of left and right channel recorded signals, the addition of an expander alters the stereo sound image produced by left and right speaker units because expansion alters the signal levels of the left and right channels to different degrees, depending on the ratio of the amplitudes of the stereo image producing signals thereof. Thus, for example, if the difference between the loudness of a louder left channel relative to the right channel is 6 db, after expansion of each channel separately the loudness difference could now be 12-20 db, and thus, the left channel would dominate to a greater extent, causing the apparent stereo image location to shift to the left. To avoid this effect and for economy sake, many expanders use only one DC control signal derived from the sum of the left and right signals to control both left and right channels. This sum signal is, by definition, a monaural signal. This method of expansion has two limitations. Firstly, since either channel controls the gain of both, if a signal is very low or absent from one side, the gain can still be raised by the signal of the other channel, thus allowing excessive noise to be heard in the absence of masking program signals. Secondly, if relatively fast time constants are used for the expansion, a reduced sense of stereo image results because the control signal is monaural.
My U.S. Pat. No. 3,980,964 describes an expander circuit which reduces distortion, noise and pumping effects while allowing fast accurate following of the program envelope. This results in a greater sense of realism than with slower designs since all aspects of the program,. including fast transients are expanded. This expander circuit, in addition to the presence of a non-frequency selective variable gain stage and a fast acting expansion control signal producing circuit which is preferably an AC to DC converter circuit controlling the same, is provided with a high pass filter which filters out low frequencies, so that the converter circuit produce DC gain control signals reflecting the amplitude of the harmonics of the fundamental frequencies of the audio signal. The commercial form of this circuit incorporated in a stereo amplifier system, before the present invention was conceived, decreased the amount of stereo signal ratio modification caused by the expansion process by cross-coupling a fraction of each DC control signal produced by each AC to DC converter circuit to the control terminal of the variable gain stage of the other channel. Thus, each channel had a small gain control effect on the other channel so that, with this cross-coupling, the stereo image retained a better resemblance to that which would be produced by the original signals. (This stereo image improvement is achieved independently of the presence of the high pass filters described, which are responsible for the distortion, noise and pumping effect reducing the advantages of this circuit).
As previously indicated, many stereo recordings fail to produce a significant apparent monaural image to give a more realistic special effect to the listener. In using the cross-coupling of DC control signals to only moderately reduce the stereo separation in the amplifier channels just described to re-establish a more accurate stereo image, many recordings still produce a relatively poor apparent monaural image. While this apparent monaural image could be increased in the circuit just described by increasing the ratio of the cross-coupled DC control signals, such an increased cross-coupling of signals required to establish a significant apparent monaural image usually undesirably reduces the desired stereo separation so that the desired stereo image is not achieved.
Accordingly, an object of the present invention is to provide expansion control circuitry which cross-couples expansion control signals in a manner which both improves the stereo and the apparent monaural images produced by the multi-channel amplifier and sound system involved. Another object of the invention is to produce such an improved expansion circuit at a modest cost.